This Leeds & Northrup precision resistance thermometer (Mueller) bridge was used by CSIRO Applied Physics to establish Australia's first complete temperature scale. This was based on the International Practical Temperature Scale developed in 1948 (IPTS-48). This first scale was used to calibrate all of the most precise temperature measurement devices used in the country.

This bridge was also used as part of their equipment in research to extend the range of use of platinum resistance thermometers up to the freezing point of silver (962 °C).

This device is an example of the highest accuracy thermometer resistance measurement device for the period 1950-1975. It was replaced by Alternating Current (AC) driven equipment, which had even greater accuracy and precision.

Leeds & Northrup made 50-60 precision resistance thermometer bridges (Mueller bridge) for use in setting temperature standards. Two units came to Australia.

The Mueller bridge (a variation on a Wheatstone bridge) is a highly accurate bridge configuration that is used to measure resistance temperature detectors (RTDs) aka platinum resistance thermometers (PRTs). The bridge can compensate for the resistance of the lead wires based on their length.

The bridge drifts with time (resistance is based on the length of wire which can change from environmental effects) and needs monthly and six-monthly calibrations. Subsequent technologies were AC instruments based on ratio transformers (based on number of turns of wire) which do not drift.

This bridge was likely used with a PRT, a DC power supply and a galvanometer. PRTs operate on the principle that electrical resistance changes in pure metal elements, relative to temperature. The bridge measures resistance to millionths of an ohm which is converted to a temperature value with a sensitivity of approximately +/-0.005 degrees centigrade.

This Leeds & Northrup precision resistance thermometer (Mueller) bridge was used by CSIRO Applied Physics in the development of an Australian temperature scale, based on the International Practical Temperature Scale. It was also used in research to extend the range of use of platinum resistance thermometers up to the freezing point of silver (962 °C).

CSIRO paid £6,000 for the bridge in 1961. Following the completion of the early research on temperature, the bridge was transferred or sold to the Defence Standards Laboratory in Adelaide, South Australia (c. 1975). This laboratory became part of the CSIRO in 1977 and was known as CSIRO Woodville North.

Precision resistance thermometer bridge, Mueller bridge, metal / plastic / glass, made by Leeds and Northrup Co, Philadelphia, United States of America, c. 1960, used by National Standards Laboratory, CSIRO, Chippendale, New South Wales, Australia, c. 1961-1975, Defence Standards Laboratory, Adelaide, South Australia, c. 1975-1977, and CSIRO, Woodville North, South Australia, c. 1977-1990

The instrument is encased in a grey metal box and has a dark grey metal face into which are set the connectors, dials, thumb wheels, switches and meter. A thermometer (missing) would have been held between the two metal fittings on the face of the unit to enable corrections for room temperature. There are two metal carry handles mounted on the face at the far right and left of the unit.

The thermometer bridge has eight decadic multiplier dials (0-11)x10, x1, x0.1, x0.01, x0.001, x0.0001, x0.00001, x0.000001; a range switch; a glass fronted meter; two large selector switches; five small dials, three thumb wheels and 21 connector posts along the top of the unit and three tap keys at the bottom next to the meter. There are connectors on the left side of the box labelled 'TEMP. CONTROL' and 'READ-OUT'.

Dials, switches, thumb wheel tops, connector tops and the meter housing are black [ebonite]. The cylindrical [ebonite] of each dial and switch is fixed into the centre of a cylinder or disk of metal onto which the position numbers are engraved. Mounted below each dial and switch is a small metal cylindrical position indicator with an engraved fiducial line.

Made: 1955 - 1965

Engraved into the metal surface of the instrument in white is:
'PRECISION RESISTANCE THERMOMETER BRIDGE
LEEDS & NORTHRUP CO.
PHILADELPHIA MADE IN U.S.A.'

Metal sticker attached to side of wood box
'THIS EQUIPMENT IS THE PROPERTY OF
COMMONWEALTH OF AUSTRALIA

PLANT
No.'

This label is engraved with a series of letters and numbers (mostly obscured by the CSIRO sticker) which appears to read
'DSL ?o ??71'

Over this metal sticker is a black dymo label typed
'C.S.I.R.O.
NM 201'

2008/174/1

1955 - 1965

250 mm

415 mm